i want to build a good accurate target AR and i dont know what is a good length barrel to use? i want at least 20" if not 22 or 24?

this will be a bench rifle and beside what length to use i need to know what twist and bullet weight i should use also?

many of you have lots of experience and i am relying on your knowledge so please help.

thanks

More velocity is better, but for 600yds and in, this'll do well: White Oak 20" 1:7 Pre-Ban

For short range (<200yds), I like 50gr VMAX. Other maglength bullets for longer distance would be the 75gr Hornady, 77gr Sierra MatchKing, 77gr Tipped Matchking, and 70gr Nosler RDF.

Longer-than-maglength, single-load bullets would be the 75gr AMAX/ELD, 80gr SMK, and 80.5gr Berger Fullbore.

Custom precision barrel maker Lilja has a line of 20" and 24" AR15 drop in match barrels.

Lilja Barrels

I have their shorter 16" Navy SEAL recon barrel.  It is 1/2 MOA capable if I don't screw up.  1:8 twist, pull button rifled, hand lapped, 416 stainless, Wylde chamber.

They can also make anything you want.

Assuming the shooter is up to it the quality of the barrel largely determines group size.
In general longer barrel equals more velocity.
Over 20” you would likely need to hand load to get the best velocity because the powder is burned up in most factory loads by 20”.
Higher velocity equals less wind drift.
In my 18” and 20” barrels I predominately shoot 77 gr SMKs with a 1:8” twist preferred.